Digital timers have been used extensively with all types of systems and devices ranging from household appliances to large size computing systems. Digital timers typically operate from a clock means driven directly by the line voltage, or can be operated from crystal controlled oscillators that are widely used in devices such as microprocessor controlled equipment. Most of the equipment that utilize digital timers do not involve safety functions, or are large enough to provide backup timers to insure proper operation. In certain types of control equipment, the redundant use of timers or backup timers can not be tolerated from a cost point of view. Certain types of this equipment, however, must be operated in a fail safe manner because of the nature of the equipment. This is particularly true of burner control equipment for programming the operation of burners that supply heat to boilers or furnaces. The failure of the timer in this type of equipment can lead to a catastrophic type of failure or explosion that has the potential of not only monetary damage, but the possible loss of life.
Most of the equipment used for timing in burner control equipment in the past has relied on a motor driven mechanism utilizing a small clock type timer that has proven to be reliable through many years of use. The clock type timer normally drives a series of cams which operate switches at appropriate time intervals so that the program contained within the burner equipment will function as programmed. This type of equipment is being rapidly replaced by digital equipment, and particularly digital equipment that includes a microprocessor for executing the control program. The operation of this new type of equipment relies on a digital timer and the failure of the digital timer clearly would create a potential failure of an unsafe nature.